Comparison of clusters with and without detectable Wolf-Rayet stars in starburst galaxies using optical, near-infrared imagery and spectroscopy

This optical, near-infrared observational study compares the properties of clusters with and without Wolf-Rayet (W-R) stars in starburst galaxies. Previous studies looked only at the galactic properties. Because this work identifies the specific locations of W-R stars, comparisons of clusters with and without W-R stars are possible and enable us to understand the impact of massive stars on their environment, to gain knowledge of the cosmic formation and evolution of chemical abundances, to constrain the parameters for modeling these systems, and to apply this knowledge to observations of starburst galaxies at higher redshifts. BHK and Ha imagery as well as spectroscopic observations over λλ3500–7000 were taken of 48 clusters in 20 galaxies. Using standard reduction techniques, we calibrated the data to provide the locations of W-R stars and to derive 35 cluster and interstellar medium properties such as age and elemental abundances. These 35 properties of clusters with and without W-R stars are compared for similarities and differences in stellar populations, nebular morphology, and elemental abundances.; Most stellar populations of clusters with and without W-R stars confirm current evolutionary theory of massive stars. Clusters with W-R stars are currently younger, bluer clusters which will later evolve into clusters without W-R stars. A few young non W-R clusters may indicate that the IMF/Upper Stellar Mass Limit is not universal; further work to determine the presence of W-R stars in these young clusters is therefore critical. Clusters with and without W-R stars show significant differences in nebular morphology, differences best explained evolutionarily. The clusters with W-R stars typically have a gas configuration similar to a Strömgren sphere so that the dust is heated, causing red H-K colors. Around the clusters without W-R stars, the gas expands into a superbubble configuration because of longer influence from stellar winds. The dust in these superbubbles obscures the stellar light, causing red B-H colors. Neither N/O nor S/O shows enhancements due to Wolf-Rayet stars. Future work will expand these studies into X-ray, ultraviolet, and infrared wavelengths.